Forty U.S. students newly arrived in Mexico, twenty U.S. students who have been in Mexico for at least one year, and twenty Mexican students (all attending classes at Universidad Autonoma de Guadalajara) will be followed for diarrhea, asymptomatic infection by enteric pathogens, and development of serologic and coproantibody changes. Additionally, 100 students in each category (newly arrived U.S., established U.S., and Mexican) will be studied intensively during the months of January and August. We will assess difference in incidence of diarrhea due to seasonality, development of immunity, and role of intervening vacation in the U.S. on duration of immunity. Seven areas will be studied in this student population: 1) The natural history of diarrheal illness and enteric infection in U.S. adults establishing residence in Mexico. 2) The temporal acquisition of immunity and serum antibody development to organism-specific infection will be characterized. 3) The relationship of different enterotoxigenic E. coli occurring over time will be determined. 4) The secretory IgA response to ETEC infection will be studied. 5) The epidemiology of travelers' diarrhea will be studied. 6) The occurrence and cause of diarrhea, experienced when U.S. persons return home after living in Mexico, will be determined. 7) Populations for future vaccine studies will be sought. Figure 4 in the research plan presents a concise overview of the study design.